Many implant systems are known for correcting orthopedic abnormalities, whether such abnormalities are naturally occurring or are due to disease or injury. For example, in cases of abnormal spinal curvature, such as scoliosis, or in cases in which one or more vertebrae must be repaired or removed, such as in tumor or fracture cases, it is common to implant a device that provides stability and support to the spine.
Two particular types of systems are well known for such repair or therapy. First, rod-based systems are known, in which a rod or bar is fixed to the spine via bone screws or hooks. Generally, the screws or hooks are implanted into one or more spinal segments, the spine is adjusted, and the rod or bar is fixed to the screws or rods to hold the spine in the corrected position. These types of systems are commonly used to correct an abnormal curvature and/or to support weak or injured vertebrae in an approximately normal curvature. One example of such a system is disclosed in U.S. Pat. No. 5,005,652 to Cotrel.
A second type of system for spinal therapy is a plate-based system. In general, a flat plate with one or more slots or holes through it is attached to one or more spinal segments by bone screws or bolts. Such plates are commonly used for the purpose of immobilizing the spinal segment(s) and promoting healing of vertebrae suffering from injury, tumor removal or other trauma or abnormality. Commonly, fusion among the vertebrae in the segment(s) is a desired therapeutic outcome, and plate-based systems are generally suited to promotion of fusion. As an example of such a plate-based system, see U.S. Pat. No. 6,315,779 to Morrison, et al., the entirety of which is incorporated herein by reference.
In some cases, plate-based systems directly attached to a bone may be incorrectly used or placed. For example, a slotted plate member that is fixed to a bone directly via a screw or bolt may fail through widening of the slot if the screw or bolt is over-tightened, or if it is placed on bone(s) in a way that could over-stress the plate. As forces from over-tightening or misplacement are transmitted to the plate, and specifically to the sides of the longitudinal slot through the plate, such forces can cause the sides of the plate to bow outward through repeated stress. The result is that the slot widens, which may allow the attaching bolt or screw head to move with respect to the slot, reducing or eliminating the immobilizing capability of the plate system.
To overcome the possibility for such failure, several different options have been proposed. In U.S. Pat. No. 5,613,967 to Engelhardt, a holding bracket is placed over the plate. As the sides of the plate experience outwardly directed forces, the bracket holds the sides of the plate so that they do not bow. In U.S. Pat. No. 6,315,779 to Morrison et al, a stabilizing member is inserted within the plate. The stabilizing member contacts the bolt or screw that is inserted into a vertebra, rather than the plate contacting the bolt or screw, and the stabilizing member cannot be bowed outward.
Some prior art plate-based systems also suffer from a risk of loosening due to backing out of a screw inserted into a vertebra. If a bone screw backs out, e.g. due to improper insertion or unanticipated stress, a plate member is not securely held in contact with the vertebra(e) so as to provide immobilizing support. Fusion, if not already completed, can be interrupted or counteracted, and injury to the vertebrae or associated tissue is possible. Use of caps or other parts over the heads of bone screws is known to help maintain the screws in their inserted positions and sustain secure contact between vertebrae and a plate. However, such devices add to the number of parts, particularly small parts, that a surgeon must handle in implanting the system.
Accordingly, there remains a need for an improved plate-based system that will address these shortcomings.